Climate, invasive species and land use drive population dynamics of a cold‐water specialist

Summary Climate change is an additional stressor in a complex suite of threats facing freshwater biodiversity, particularly for cold‐water fishes. Research addressing the consequences of climate change on cold‐water fish has generally focused on temperature limits defining spatial distributions, largely ignoring how climatic variation influences population dynamics in the context of other existing stressors. We used long‐term data from 92 populations of bull trout Salvelinus confluentus – one of North America's most cold‐adapted fishes – to quantify additive and interactive effects of climate, invasive species and land use on population dynamics (abundance, variability and growth rate). Populations were generally depressed, more variable and declining where spawning and rearing stream habitat was limited, invasive species and land use were prevalent and stream temperatures were highest. Increasing stream temperature acted additively and independently, whereas land use and invasive species had additive and interactive effects (i.e. the impact of one stressor depended on exposure to the other stressor). Most (58%–78%) of the explained variation in population dynamics was attributed to the presence of invasive species, differences in life history and management actions in foraging habitats in rivers, lakes and reservoirs. Although invasive fishes had strong negative effects on populations in foraging habitats, proactive control programmes appeared to effectively temper their negative impact. Synthesis and applications . Long‐term demographic data emphasize that climate warming will exacerbate imperilment of cold‐water specialists like bull trout, yet other stressors – especially invasive fishes – are immediate threats that can be addressed by proactive management actions. Therefore, climate‐adaptation strategies for freshwater biodiversity should consider existing abiotic and biotic stressors, some of which provide potential and realized opportunity for conservation of freshwater biodiversity in a warming world.